import java.util.*;

public class Solution3 {
    Map<Character, Set<Character>> edges; // 邻接表
    Map<Character,Integer> in; // 统计每个节点的入度
    boolean check;
    public String alienOrder(String[] words) {
        in = new HashMap<>();
        edges = new HashMap<>();
        // 1. 初始化入度哈希表 + 建表
        for(String s : words) {
            for(int i = 0; i < s.length(); i++) {
                char ch = s.charAt(i);
                in.put(ch, 0);
            }
        }

        int n = words.length;
        for(int i = 0; i < n; i++) {
            for(int j = i + 1; j < n; j++) {
                add(words[i], words[j]);
                if(check == true) {
                    return "";
                }
            }
        }

        // 2. 拓扑排序
        Queue<Character> q = new LinkedList<>();
        for(char ch : in.keySet()) {
            if(in.get(ch) == 0) {
                q.add(ch);
            }
        }
        StringBuffer ret = new StringBuffer();
        while(!q.isEmpty()) {
            char t = q.poll();
            ret.append(t);
            if(!edges.containsKey(t)) {
                continue;
            }
            for(char ch : edges.get(t)) {
                in.put(ch, in.get(ch) - 1);
                if(in.get(ch) == 0) {
                    q.add(ch);
                }
            }
        }
        // 3. 判断
        for(char ch : in.keySet()) {
            if(in.get(ch) != 0) {
                return "";
            }
        }
        return ret.toString();
    }

    public void add(String s1, String s2) {
        int n = Math.min(s1.length(), s2.length());
        int i = 0;
        for( ; i < n; i++) {
            char c1 = s1.charAt(i);
            char c2 = s2.charAt(i);
            if(c1 != c2) {
                // c1 - > c2
                if(!edges.containsKey(c1)) {
                    edges.put(c1, new HashSet<>());
                }
                if(!edges.get(c1).contains(c2)) {
                    edges.get(c1).add(c2);
                    in.put(c2, in.get(c2) + 1);
                }
                break;
            }
        }
        if(i == s2.length() && i < s1.length()) {
            check = true;
        }
    }
}
